Conventionally, as that which determines a sleep stage of a subject, that described in Japanese Unexamined Patent Application Publication No. 2005-152310 is known, which estimates a sleep stage based on the biological information about heartbeat fluctuation measured by a flow velocity or flow volume sensor of a fluid that flows through a vent section provided in an air mat, and body movement information. The purpose of the estimation device and the estimation method is to estimate a sleep stage of a subject who lies down on an air mat in an unrestrained state.